The present invention relates broadly to thermal management materials for electronic devices. Such materials commonly are used as heat transfer interfaces between, for example, the mating heat transfer surfaces of a heat-generating, electronic component, such as an integrated circuit (IC) chip, and a thermal dissipation member, such as a heat sink or spreader, for the conductive cooling of the electronic component. More particularly, the present invention relates to a laminar, thermally-conductive interface which is formed as a laminate of a first layer of a conformable phase-change material and a second layer of a flexible, lamellar graphite material or, alternatively, a tin foil material. The first layer, which may be formulated to be inherently tacky or otherwise adherable to the heat transfer surface of the dissipation member, is form stable in a first phase at normal room temperature but conformable to the heat transfer surface in a second phase within the operating temperature of the electronic component to provide a low thermal impedance interface with the dissipation member. The second layer, in turn, is relatively compressible and thereby also relatively conformable to the heat transfer surface of the electronic component to provide a low thermal impedance interface therewith. However, the second layer is cleanly removable, i.e., without substantial residue, from the surface of the component for rework or otherwise.
Circuit designs for modem electronic devices such as televisions, radios, computers, medical instruments, business machines, communications equipment, and the like have become increasingly complex. For example, integrated circuits have been manufactured for these and other devices which contain the equivalent of hundreds of thousands of transistors. Although the complexity of the designs has increased, the size of the devices has continued to shrink with improvements in the ability to manufacture smaller electronic components and to pack more of these components in an ever smaller area.
As electronic components have become smaller and more densely packed on integrated boards and chips, designers and manufacturers now are faced with the challenge of how to dissipate the heat which is ohmicly or otherwise generated by these components. Indeed, it is well known that many electronic components, and especially power semiconductor components such as transistors and microprocessors, are more prone to failure or malfunction at high temperatures. Thus, the ability to dissipate heat often is a limiting factor on the performance of the component.
Electronic components within integrated circuits traditionally have been cooled via forced or convective circulation of air within the housing of the device. In this regard, cooling fins have been provided as an integral part of the component package or as separately attached thereto for increasing the surface area of the package exposed to convectively-developed air currents. Electric fans additionally have been employed to increase the volume of air which is circulated within the housing. For high power circuits and the smaller but more densely packed circuits typical of current electronic designs, however, simple air circulation often has been found to be insufficient to adequately cool the circuit components.
Heat dissipation beyond that which is attainable by simple air circulation may be effected by the direct mounting of the electronic component to a thermal dissipation member such as a xe2x80x9ccold platexe2x80x9d or other heat sink or spreader. The dissipation member may be a dedicated, thermally-conductive ceramic or metal plate or finned structure, or simply the chassis or circuit board of the device. However, beyond the normal temperature gradients between the electronic component and the dissipation member, an appreciable temperature gradient is developed as a thermal interfacial impedance or contact resistance at the interface between the bodies.
That is, and as is described in U.S. Pat. No. 4,869,954, the faying thermal interface surfaces of the component and heat sink typically are irregular, either on a gross or a microscopic scale. When the interfaces surfaces are mated, pockets or void spaces are developed therebetween in which air may become entrapped. These pockets reduce the overall surface area contact within the interface which, in turn, reduces the heat transfer area and the overall efficiency of the heat transfer through the interface. Moreover, as it is well known that air is a relatively poor thermal conductor, the presence of air pockets within the interface reduces the rate of thermal transfer through the interface.
To improve the heat transfer efficiency through the interface, a pad or other layer of a thermally-conductive, electrically-insulating material typically is interposed between the heat sink and electronic component to fill in any surface irregularities and eliminate air pockets. Initially employed for this purpose were materials such as silicone grease or wax filled with a thermally-conductive filler such as aluminum oxide. Such materials usually are semi-liquid or solid at normal room temperature, but may liquefy or soften at elevated temperatures to flow and better conform to the irregularities of the interface surfaces.
The greases and waxes of the aforementioned types heretofore known in the art, however, generally are not self-supporting or otherwise form stable at room temperature and are considered to be messy to apply to the interface surface of the heat sink or electronic component. To provide these materials in the form of a film which often is preferred for ease of handling, a substrate, web, or other carrier must be provided which introduces another interface layer in or between which additional air pockets may be formed. Moreover, use of such materials typically involves hand application or lay-up by the electronics assembler which increases manufacturing costs.
Alternatively, another approach is to substitute a cured, sheet-like material in place of the silicone grease or wax. Such materials may be compounded as containing one or more thermally-conductive particulate fillers dispersed within a polymeric binder, and may be provided in the form of cured sheets, tapes, pads, or films. Typical binder materials include silicones, urethanes, thermoplastic rubbers, and other elastomers, with typical fillers including aluminum oxide, magnesium oxide, zinc oxide, boron nitride, and aluminum nitride.
Exemplary of the aforesaid interface materials is an alumina or boron nitride-filled silicone or urethane elastomer which is marketed under the name CHO-THERM(copyright) by the Chomerics TEC Division of Parker-Hannifin Corp., 16 Flagstone Drive, Hudson, N.H. 03051. Additionally, U.S. Pat. No. 4,869,954 discloses a cured, form-stable, sheet-like, thermally-conductive material for transferring thermal energy. The material is formed of a urethane binder, a curing agent, and one or more thermally conductive fillers. The fillers, which may include aluminum oxide, aluminum nitride, boron nitride, magnesium oxide, or zinc oxide, range in particle size from about 1-50 microns (0.05-2 mils).
Sheets, pads, and tapes of the above-described types have garnered general acceptance for use as interface materials in the conductive cooling of electronic component assemblies such as the semiconductor chips, i.e., dies, described in U.S. Pat. No. 5,359,768. In certain applications, however, heavy fastening elements such as springs, clamps, and the like are required to apply enough force to conform these materials to the interface surfaces in order to attain enough surface for efficient thermal transfer. Indeed, for some applications, materials such as greases and waxes which liquefy, melt, or soften at elevated temperature continue to be preferred as better conforming to the interface surfaces under relatively low clamping pressures.
Recently, phase-change materials have been introduced which are self-supporting and form-stable at room temperature for ease of handling, but which liquefy or otherwise soften at temperatures within the operating temperature range of the electronic component to form a viscous, thixotropic phase which better conforms to the interface surfaces. These phase-change materials, which may be supplied as free-standing films, or as heated screen printed onto a substrate surface, advantageously function much like greases and waxes in conformably flowing within the operating temperature of the component under relatively low clamping pressures of about 5 psi (35 kPa). Such materials are further described in commonly-assigned U.S. Pat. No. 6,054,198, and are marketed commercially under the names THERMFLOW(trademark) T310, T443, T705, T710, T725, and A725 by the Chomerics TEC Division of Parker-Hannifin Corp., 16 Flagstone Drive, Hudson, N.H. 03051. Other phase-change materials are marketed commercially by the Bergquist Company (Minneapolis, Minn.) under the tradename xe2x80x9cHI-FLOW(trademark),xe2x80x9d by Thermagon, Inc. (Cleveland, Ohio) under the tradenames xe2x80x9cT-PCM(trademark)xe2x80x9d and by Orcus, Inc. (Stilwell, Kans.) under the tradename xe2x80x9cTHERMAPHASE.xe2x80x9d A phase-change material/metal foil laminate is marketed by Thermagon, Inc. under the tradename xe2x80x9cT-MATE(trademark).xe2x80x9d
For typical commercial application, the thermal interface material may be supplied in the form of a tape or sheet which includes an inner and outer release liner and an interlayer of thermal compound. Unless the thermal compound is inherently tacky, one side of the compound layer may be coated with a thin layer of a pressure-sensitive adhesive (PSA) for the application of the compound to the heat transfer surface of a heat sink. In order to facilitate automated dispensing and application, the outer release liner and compound interlayer of the tape or sheet may be die cut to form a series of individual, pre-sized pads. Each pad thus may be removed from the inner release liner and bonded to the heat sink using the adhesive layer in a conventional xe2x80x9cpeel and stickxe2x80x9d application which typically is performed by the heat sink manufacturer.
With the pad being adhered to the heat transfer surface of the thermal dissipation member such as a heat sink or spreader, and with the outer liner in place to form a protective cover the outer surface of the compound layer, the dissipation member and pad may be provided as an integrated assembly. Prior to installation of the assembly, the outer release liner is removed from the compound layer, and the pad positioned on the electronic component. A clamp may be used to secure the assembly in place.
It will be appreciated, however, that further improvements in thermal interfaces and in the materials therefor would be well-received by the electronics industry. In this regard, certain applications may dictate that the interface be cleanly removable, i.e., without substantial residue of its constituent materials, from the surface of the electronic component for rework, repositioning, or other disassembly. Especially desired therefore would be a thermal interface pad, sheet, or the like which is conformable upon being heated in service to provide a low thermal impedance but which is also cleanly releasable after service from the surface of the electronic component for rework, repositioning, or other disassembly.
The present invention is directed to a thermal interface for a thermal management assembly involving, for example, a heat source such as an electronic chip or other heat-generating component and a thermal dissipation member such as a heat sink or spreader disposable in thermal adjacency with the electronic component for the conduction of heat therebetween. Particularly, the invention is directed to an interface, such as in the form of a sheet or pad, which is conformable to the interfacing surface of the electronic component and the heat sink or spreader to provide a low thermal impedance across the assembly, but which also is cleanly releasable from the electronic component for rework, repositioning, or other disassembly.
Heretofore, interfaces of the type herein involved formed of materials of the phase-change variety generally were not specified for applications dictating a clean removal from the electronic component. In this regard, as a result of the flow of the phase-change material when the interface is heated to a temperature within the operating temperature of the electronic component, the interface was not able to be cleanly released from the component. Rather, solvents or mechanical means such as scraping were required to be employed to clean the component for rework or the like.
The interface of the present invention, in being formed as a laminate of a first layer formed of a flexible, lamellar graphite material or, alternatively, a tin foil material, disposable in heat transfer contact with a heat transfer surface of the electronic component, and a second layer formed of a phase-change material disposable in heat transfer contact with a heat transfer surface of a dissipation member such as a heat sink or spreader, is both conformable to the surfaces of the dissipation member and the component. However, as a result of the first layer being formed of the graphite or tin material which does not substantially flow under the temperatures and pressures herein involved, the interface therefore is cleanly removable from the surface of the electronic component without resort to chemical or mechanical means.
Advantageously, the interface of the present invention may be removed with the heat sink or spreader from heat transfer contact with the electronic component without having to wait for the component to cool, thus speeding manufacturing and rework such as in the case of a problem with the initial burn-in of a microprocessor. Moreover, even after heating, the component and heat sink or spreader may be separated easily with little or no force with the attendant elimination of any substantial stress that otherwise would be placed on the component. In contrast, even if conventional PCM""s could be released cleanly from the component, appreciable force nonetheless is required to break the vacuum developed as the PCM wets the surface of the component.
Unexpectedly, it has been observed that a layer of a flexible, lamellar graphite material may be used in conjunction with a phase-change material (PCM) to provide a very low thermal impedance, but cleanly releasable interface. In this regard, it is speculated that the flexible graphite material exhibits some degree of compressibility to allow it to better conform to the surface of the electronic component and to thereby lower the thermal contact resistance therebetween with the overall effect of improved heat transfer performance as compared to a metal-PCM laminate.
Alternatively, a thin layer of a tin foil material may be substituted for the graphite material. As compared to metal-PCM laminates conventionally based on aluminum foil materials, the tin-PCM laminates of the present invention exhibits a markedly lower thermal impedance and correspondingly improved heat transfer performance. Such improved performance is believed to be surprising insofar as the thermal conductivities of aluminum materials are generally known and reported to be higher than those for tin materials, e.g., about 200 W/m-K for aluminum versus about 60 W/m-K for tin. Thus, aluminum-PCM laminates generally would be expected to out-perform tin-PCM laminates in the thermal interface applications herein involved. Experimental testing, however, has revealed the opposite to be true. As additional advantages, by using thin gauge tin foils, the tin-PCM laminates of the present invention may be provided to be relatively thin and flexible, and are easier to manufacture and may be produced at a reduced cost as compared to other materials.
In an illustrative embodiment, the PCM of the second layer is form-stable in a first phase at normal room temperature but conformable, as being melted or otherwise softened, to the heat transfer surface in a second phase at an elevated temperature or range within the operating temperature of the electronic component to provide a low thermal impedance contact with the dissipation member even under relatively low clamping forces. Particularly, the PCM may be formulated as an admixture of a polymeric component and one or more thermally-conductive fillers. The polymeric component may be a resin such as a pressure-sensitive adhesive (PSA) or thermoplastic hot-melt, a paraffinic or other wax, a blend of one or more resins or one or more waxes, or a blend of one or more resins and one or more waxes. Preferably, the polymeric component is formulated to be inherently tacky to enable the interface to be bondable at room temperature to the surface of the heat sink, spreader, or the like without the necessity of heating or the provision of a separate pressure-sensitive adhesive (PSA) or other adhesive layer. The thermally-conductive filler, which may be provided as flakes, fibers, particles, or other particulate loaded at between 20-80% by weight, may be boron nitride, titanium diboride, aluminum nitride, silicon carbide, graphite, a metal such as silver, aluminum or copper, a metal oxide such as aluminum oxide, magnesium oxide, zinc oxide, beryllium oxide, or antimony oxide, or a mixture thereof. The flexible, lamellar graphite material of the first layer, in turn, may be intercalated graphite flakes which are heated and then molded, calendered, or otherwise formed into a sheet. The PCM may be coated onto a sheet of the graphite or tin foil materials herein involved, or otherwise bonded thereto by laminating under conditions of elevated heat and/or pressure.
The present invention, accordingly, comprises the combination of elements and arrangement of parts and steps which are exemplified in the detailed disclosure to follow. Advantages of the present invention include a thermal interface which is conformable for lower contact resistance and more efficient heat transfer between, for example, an electronic component and a heat sink, spreader, or other dissipation member within a thermal management assembly, but which also is cleanly releasable from the surface of the electronic component for rework, repositioning, or other disassembly. Further advantages include a thermal interface including a PCM which may be supplied in the form of a sheet, tape, or pad, and which is form-stable at room temperature for ease of handling and use. Still other advantages include a thermal interface material having a clean break capability from the electronic component which is not temperature dependent. These and other advantages will be readily apparent to those skilled in the art based upon the disclosure contained herein.